4FMFES-PET Imaging of ER+ Advanced Breast Cancers

Estrogen Receptor (ER) is a crucial prognostic factor and treatment target in breast cancer patients. Knowledge of its status greatly influences the choice of the optimal course of treatment. Pathological evaluations of primary tumor, axillary nodes, and metastases are the only confirmatory approach to ER status determination and are limited to known and accessible sites. However, it is known that many advanced breast cancer patients harbor diseases presenting inter-tumor or temporal ER heterogeneity, as ER expression can vary between tumor foci and can evolve during treatment and at time of recurrence, hence the need for whole-body, non-invasive assessment of ER status. In the last decades, 16α-\[18F\]fluoroestradiol (FES) was developed and evaluated as an ER-targeting positron emission tomography (PET) tracer. FES correlated with ER expression, and recently was shown to be able to predict hormone therapy response. Our Center designed and evaluated 4-fluoro-11β-methoxy-16α-\[18F\]fluoroestradiol (4FMFES), a successor PET tracer for ER imaging. Paired comparison during a phase II clinical trial showed that 4FMFES produced images of better quality, with less overall non-specific signal than FES. It resulted in a significantly improved tumor contrast and tumor detectability using 4FMFES-PET leading to increased diagnosis confidence in early-stage breast cancer compared to FES-PET. Those results demonstrated that, as of now, 4FMFES-PET is the best imaging modality worldwide for whole-body ER status determination, but further validations are necessary to position this method as a standard and essential tool for breast cancer management. Like what was observed for FES-PET, preliminary data suggest that 4FMFES-PET combined with FDG-PET will yield very high sensibility for breast tumor detection, each method being complementary. In continuity with previous work, we seek to expand our clinical knowledge of this high-potential diagnostic imaging through the following main